1. Field of the Invention
This invention relates to a linear damper exerting an effective damping force without the use of fluid.
2. Description of the Related Art
FIG. 57 illustrates a conventionally known linear damper exerting a damping effect on movement in the axial direction.
The conventional linear damper includes a cylindrical-shaped casing 1, a piston 2 sliding in contact with the inner surface of the casing 1, and a rod 3 connected to the piston 2. The casing 1 has an open end 1a and a closed end 1b. The open end 1a is covered with a cap 4 for enclosure.
The piston 2 divides the inside of the enclosed casing 1 into two chambers 5a and 5b which contain viscous fluid.
The casing 1 has a groove 6 formed on the inner peripheral surface thereof. The groove 6 formed serves as a passageway allowing the flow of the viscous fluid along the sliding surface between the casing 1 and the piston 2.
The cap 4 has a rod hole 4a that supports the rod 3 protruding from casing 1 through the cap 4 to the outside.
An external force pushes the rod 3 against the spring force of a spring 7, whereupon the piston 2 moves downward in the casing 1. Upon release of the external force, the spring force of the spring 7 moves the piston 2 upward in the casing 1.
Such a downward movement of the piston 2 causes the viscous fluid to move from the chamber 5a through the groove 6 into the chamber 5b. The resistance of the viscous fluid produced at this point activates the damping effect.
An accumulator 8 is provided in the chamber 5b for absorbing the fluid up to an amount equivalent to the volume of the rod 3.
In the conventional damper described above as an example, because the casing 1 holds viscous fluid, there is a need to seal the area between the rod 3 and the rod hole 4a with a seal member (not shown) for the prevention of leakage of the fluid.
The rod 3 slides inside the rod hole 4a with one end thereof fixed to the piston 2. Therefore, the exacting dimensions of both the rod 3 and the rod hole 4a must be determined with extreme precision for the smooth sliding of the rod 3 in the rod hole 4a. 
Such a linear damper is also described in Japanese unexamined patent publication No. 3-277839 (page 2 and FIG. 2).
As described hitherto, the conventional linear damper is an oil damper of which the casing 1 accommodates viscous fluid and the flow resistance of the viscous fluid is used to produce the damping effect. For this reason, oil is absolutely necessary. This brings with it the need to provide a seal member for preventing the leakage of fluid. But however tight the seal is applied, the oil adhering to the rod or the like will inevitably leak to the outside. A complete elimination of oil leakage is virtually impossible in reality. Such characteristics of the oil damper produce the problem of the impossibility of using the oil damper in a situation where the adhesion of the oil to food or the like must be absolutely avoided.
Although the complete elimination of oil leakage is close to impossible in reality, in order to approach a complete elimination as closely as possible, there is a need to increase the precision of the seal structure. In this respect, however, the problem is that the higher the precision of the seal structure, the higher the cost.
If the seal function is to be performed satisfactorily without an increase in the seal precision, an increase in the tightening force of the seal is needed. However, as the tightening force of the seal becomes increased, the friction increases. This in turn causes impairment in the sliding performance of the rod and thus adversely affects the damping effect.
Further, a seal groove is required to hold the seal member, but the formation of the seal groove in itself requires time and effort. This also becomes a factor in increasing the cost.
For the prevention of oil leakage and a reduction in slide resistance to a minimum, moreover, the surface of the rod 3 is required to be finished with a high degree of accuracy, naturally resulting in an increase in the cost. In addition, another problem is that as compared with the machining techniques for resin and the like, metal work requires time and effort, so that the higher the degree of accuracy of machining metal is required, the higher the cost.
Whatever the case, the conventional oil dampers face the inescapable problems of not only a limited use but also a substantial increase in production costs.
On the other hand, an air damper having a cylinder filled with a gas is conventionally known as a damper compensating for the drawback, e.g. the oil leakage, in the oil dampers as described above. However, if the gas leaks, it becomes next to impossible to expect a damping effect from the air damper. In addition, to completely prevent the leakage of a gas composed of extremely small particles is much more difficult than to block the oil leakage.
As a result, though the air damper has no structural problems, the problem is that the air damper becomes functionally unusable due to the gas leakage.
Moreover, due to the high degree of compressibility of the gas or the like, the air damper has the characteristic of response inferior to that of the oil damper.
In other words, the oil damper is capable of having a long life to a certain extent, and promising to offer a strong damping force, but has the problem of the impossibility of being used in a situation where oil leakage is forbidden. The air damper has no problems in respect to oil leakage, but has the problems of a short life and a somewhat inadequate response.